Boronic Acid-Containing Macroscopic Hydrogels

These synthesis routes produce hydrogels with pendant PBA groups. Because of the PBA groups, these hydrogels exhibit glucose-dependent swelling behavior, which is a major property of these gels. There exists an 

The formation of a hydrogel upon mixing a poly(vinyl alcohol) (PVA) solution with a borax solution was reported more than six decades ago. Many other polyhydroxy polymers, such as poly(glyceryl methacrylate) and some polysaccharides,can also be gelled by borax. NMR spectroscopy studies indicated that the B(OH)4 anion binds to alternate OH groups on the polymer, especially when they are in the meso configuration. The nature of the crosslink is not absolutely clear, as there are proposals for the borate ion to link to both chains directly, while others have suggested that one of the links is via the sodium ion.  

In these hydrogels, the number of crosslinks is determined by the ehemi-cal equilibrium among boric acid, borate ions, and diol sites on the polymer chains, which can be influenced by both temperature and pH. Therefore, pH and temperature can dramatically affect the viscoelastic behavior of the borate-polyol gels. The sol-gel transition typically occurs at pH 8-9, near the pAa of the boron compound. The formation of the crosslink is exothermic, between 1-2 kj mol so that gel formation can be opposed by heating.  

Similar to borax, boronic acid-containing polymers can also be used to crosslink polyhydroxy polymers, such as PVA. Kitano et al. reported the first example of an interpolymer complex based on boronate-diol interactions. The complex was formed by mixing a PVA solution and an alkali solution of poly(AI-vinyl-pyrrolidone-co-3-acrylamidophenylboronic acid). Complex formation leads to an increase in solution viscosity. Above a critical polymer concentration, the complex solution loses its fluidity to become a transparent gel. The authors later increased the solubility of the polymer under physiological and acidic aqueous conditions by incorporation a third comonomer, Af,Af-dimethylaminopropylacrylamide (DMAPAA). Therefore, an interpolymer complex can form at physiological pH. 

Compared with borax, boronic acid-containing polymers are more effective crosslinkers for PVA, because the polymer crosslinkers allow for multipoint interaction between them and PVA. Ivanov et used 

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